Device and method for modifying actuation voltage thresholds of a deformable membrane in an interferometric modulator

ABSTRACT

By varying the spacing between a partially-reflective, partially-transmissive surface and a highly reflective surface positioned behind the partially-reflective, partially-transmissive surface, an interferometric modulator selectively creates constructive and/or destructive interference between light waves reflecting off the two surfaces. The spacing can be varied by applying a voltage to create electrostatic attraction between the two surfaces, which causes one or both surfaces to deform and move closer together. In the absence of such attraction, the surfaces are in a relaxed position, where they are farther apart from one another. A actuation voltage is needed to create sufficient electrostatic attraction to cause a surface to deform. The actuation voltage can be modified by implanting ions in a dielectric layer attached to one or both surfaces. Upon the application of a voltage, the ions create a baseline level of repulsion or attraction between the two surfaces, which thus require more or less voltage, respectively, to cause a surface to deform. The degree of ion implantation can be chosen to set the actuation voltage as desired, or the surfaces can be made to deform at a given voltage by appropriately selecting the degree of ion implantation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application No. 60/613,451, filed Sep. 27, 2004. Thisapplication is also related to U.S. patent application Ser. No.10/251,196, filed Sep. 20, 2002; and U.S. patent application Ser. No.11/090,911, filed Mar. 25, 2005.

BACKGROUND

1. Field of the Invention

This invention relates to microelectromechanical systems (MEMS) and,more particularly, to devices and methods for selectively creatingconstructive and/or destructive interference of light waves.

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF CERTAIN EMBODIMENTS

In accordance with one aspect of the invention, a microelectromechanicalsystem is provided. The microelectromechanical system comprises aconductor layer, a mechanical layer and a charged layer. The mechanicallayer is separated by a cavity from the conductor layer and isconfigured to move relative to the conductor layer. The charged layercomprises an incorporated charged species and is disposed between theconductor layer and the mechanical layer.

In accordance with another aspect of the invention, a method is providedfor modulating electromagnetic radiation. The method comprises providinga plurality of micromechanical devices. Each device comprises aconductor layer, a reflective layer and a charged layer between theconductor layer and the reflective layer. The charged layer has anincorporated charged species. The reflective layer is parallel to andspaced a distance from the conductor layer while in a relaxed state. Thedistance for some of the micromechanical devices is different from otherof the micromechanical devices. The reflective layer is configured tomove relative to the conductor layer upon being switched to an actuatedstate. A voltage is applied to the conductor layers and the reflectivelayers of the micromechanical devices to actuate the micromechanicaldevices.

In accordance with yet another aspect of the invention, a method isprovided for fabricating a micromechanical device. The method comprisesforming a first conductive layer, forming a dielectric layer over thefirst conductive layer, adding charge to the dielectric layer andforming a second conductive layer over the dielectric layer. In anotheraspect, the invention provides a micromechanical device formed by thismethod.

In accordance with another aspect of the invention, an interferometricmodulator is provided. The modulator comprises a conductor layer, amovable layer and a means for modifying a voltage actuation threshold ofthe movable layer with charged species.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a cross section of an interferometric modulator having anincorporated charged species.

FIG. 9 is a cross section of another interferometric modulator having anincorporated charged species.

FIG. 10 is a cross section of yet another interferometric modulatorhaving an incorporated charged species.

FIGS. 11-12 illustrate the effect of charge incorporation on thehysteresis window of interferometric modulators.

FIG. 13 illustrates a cross section of additional alternativeembodiments of interferometric modulators.

FIG. 14 illustrates the effect of charge incorporation on the hysteresiswindows of interferometric modulators of FIG. 13.

FIGS. 15-16 are flowcharts showing steps in processing sequences formaking an interferometric modulator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As discussed in greater detail below, interferometric modulators can beswitched between a bright and a dark state by moving a reflective part(the “reflective layer”) relative to a partly-transmissive andpartly-reflective part (the “optical stack”), which is spaced from thereflective layer. The movement is actuated by creating electrostaticattraction between the two parts, which causes at least one of the partsto move relative to the other part. In an actuated position, one of theparts has a net positive charge, while the other part has a net negativecharge, thereby causing the parts to be drawn close together. In arelaxed position, the net charge between the parts is not sufficient toovercome the mechanical resistance of the parts to movement, and theparts are spaced relatively far apart. The voltage needed to generatesufficient electrostatic attraction to draw the parts into the actuatedposition may be referred to as the actuation voltage.

According to some preferred embodiments, the actuation voltage can bealtered by incorporating positively and/or negatively charged species,such as ions, into the reflective layer and/or the optical stack. Thereflective layer and/or the optical stack are preferably provided with adielectric layer, situated between the reflective layer and the opticalstack, into which the charged species can be embedded. The chargedspecies create a constant, baseline level of charge, which augmentand/or cancel part of the electrostatic attraction that is generatedwhen applying a voltage to the optical stack and the reflective layer.As a result, a higher or lower actuation voltage may be needed togenerate the net level of electrostatic attraction necessary to, e.g.,cause the reflective layer to move to an actuated position. For example,if the reflective layer and the optical stack are wired so as to have apositive and a negative charge, respectively, then the actuation voltagecan be increased by implanting positively charged ions, which can repelthe positively charged reflective layer. Conversely, the actuationvoltage can be decreased by implanting negatively charged ions, whichhelp to attract the positively charged reflective layer. Thus, theincorporation of the charged species can be used to alter the actuationvoltage as desired.

It will also be appreciated that, as discussed further below, theinterferometric modulators exhibit a hysteresis behavior in which theyremain in a particular state over a range of applied voltages. Thisrange of applied voltages is referred to as the “hysteresis window.” Forexample, the interferometric modulator remains stable in the relaxedposition until the applied voltage is increased to the actuationvoltage, when it switches to the actuated position. The interferometricmodulator then remains stable in the actuated state until the appliedvoltage drops below a certain voltage. Pre-charging, e.g., by ionimplantation preferably leaves the hysteresis window substantiallyunchanged and shifts the window with the actuation voltage.Advantageously, this shifting allows the windows to be centered asdesired, allowing for the simplification of driver and control systems,as discussed below.

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12, which may be referred to separately as 12a and 12 b. The interferometric modulators 12 each include a movablemechanical layer 14, which is preferably reflective, and an opticalstack 16, which may be referred to separately as movable reflectivelayers 14 a and 14 b and optical stacks 16 a and 16 b. In theinterferometric modulator 12 a on the left, the movable reflective layer14 a is illustrated in a relaxed position at a predetermined distancefrom the optical stack 16 a, which includes a partially reflectivelayer. In the interferometric modulator 12 b on the right, the movablereflective layer 14 b is illustrated in an actuated position adjacent tothe optical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise of several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. In some embodiments, the layers are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective material,such as aluminum, may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,in the actuated position, as illustrated by pixel 12 b on the right inFIG. 1. The behavior is the same regardless of the polarity of theapplied potential difference. In this way, row/column actuation that cancontrol the reflective vs. non-reflective pixel states is analogous inmany ways to that used in conventional LCD and other displaytechnologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a panel or display array (display) 30. The cross section ofthe array illustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. ForMEMS interferometric modulators, the row/column actuation protocol maytake advantage of a hysteresis property of these devices illustrated inFIG. 3. It may require, for example, a 10 volt potential difference tocause a movable layer to deform from the relaxed state to the actuatedstate. However, when the voltage is reduced from that value, the movablelayer maintains its state as the voltage drops back below 10 volts. Inthe exemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the relaxed or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be relaxed areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or relaxed pre-existingstate. Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Relaxing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, it will be appreciated that voltages of opposite polarity than thosedescribed above can be used, e.g., actuating a pixel can involve settingthe appropriate column to +V_(bias), and the appropriate row to −ΔV. Inthis embodiment, releasing the pixel is accomplished by setting theappropriate column to −V_(bias), and the appropriate row to the same−ΔV, producing a zero volt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, a microphone 46, and an input device 48. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding, and vacuum forming. In addition, the housing 41 may be madefrom any of a variety of materials, including but not limited toplastic, metal, glass, rubber, and ceramic, or a combination thereof. Inone embodiment the housing 41 includes removable portions (not shown)that may be interchanged with other removable portions of differentcolor, or containing different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to the processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to the arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one oremore devices over a network. In one embodiment the network interface 27may also have some processing capabilities to relieve requirements ofthe processor 21. The antenna 43 is any antenna known to those of skillin the art for transmitting and receiving signals. In one embodiment,the antenna transmits and receives RF signals according to the IEEE802.11 standard, including IEEE 802.11(a), (b), or (g). In anotherembodiment, the antenna transmits and receives RF signals according tothe BLUETOOTH standard. In the case of a cellular telephone, the antennais designed to receive CDMA, GSM, AMPS or other known signals that areused to communicate within a wireless cell phone network. Thetransceiver 47 pre-processes the signals received from the antenna 43 sothat they may be received by and further manipulated by the processor21. The transceiver 47 also processes signals received from theprocessor 21 so that they may be transmitted from the exemplary displaydevice 40 via the antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

The processor 21 generally controls the overall operation of theexemplary display device 40. The processor 21 receives data, such ascompressed image data from the network interface 27 or an image source,and processes the data into raw image data or into a format that isreadily processed into raw image data. The processor 21 then sends theprocessed data to the driver controller 29 or to frame buffer 28 forstorage. Raw data typically refers to the information that identifiesthe image characteristics at each location within an image. For example,such image characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, the driver controller29 is a conventional display controller or a bi-stable displaycontroller (e.g., an interferometric modulator controller). In anotherembodiment, the array driver 22 is a conventional driver or a bi-stabledisplay driver (e.g., an interferometric modulator display). In oneembodiment, the driver controller 29 is integrated with the array driver22. Such an embodiment is common in highly integrated systems such ascellular phones, watches, and other small area displays. In yet anotherembodiment, the display array 30 is a typical display array or abi-stable display array (e.g., a display including an array ofinterferometric modulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

The power supply 50 can include a variety of energy storage devices asare well known in the art. For example, in one embodiment, the powersupply 50 is a rechargeable battery, such as a nickel-cadmium battery ora lithium ion battery. In another embodiment, power supply 50 is arenewable energy source, a capacitor, or a solar cell, including aplastic solar cell, and solar-cell paint. In another embodiment, thepower supply 50 is configured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. Those of skill in the art will recognizethat the above-described optimization may be implemented in any numberof hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports 18 at thecorners only, on tethers 32. In FIG. 7C, the functions of movement andreflectivity are separated. The moveable reflective layer 14 issuspended from a deformable layer 34, which may comprise a flexiblemetal. The deformable layer 34 connects, directly or indirectly, to thesubstrate 20 around the perimeter of the deformable layer 34. Theseconnections are herein referred to as support posts 18. The deformablelayer 34 constitutes the mechanical layer and the layer 14 is thereflective surface. In the embodiment illustrated in FIG. 7D the supportposts 18 include support post plugs 42 upon which the deformable layer34 rests. The movable reflective layer 14 remains suspended over thecavity, as in FIGS. 7A-7C, but unlike FIGS. 7B-7C, the deposition of thedeformable layer 34 does not form the support posts by filling holesbetween the deformable layer 34 and the optical stack 16. Rather, thesupport posts 18 are at least partially formed of a separately depositedplanarization material, which is used to form support post plugs 42. Theembodiment illustrated in FIG. 7E is based on the embodiment shown inFIG. 7D, but may also be adapted to work with any of the embodimentsillustrated in FIGS. 7A-7C as well as additional embodiments not shown.In the embodiment shown in FIG. 7E, an extra layer of metal or otherconductive material has been used to form a bus structure 44. Thisallows signal routing along the back of the interferometric modulators,eliminating a number of electrodes that may otherwise have had to beformed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged (i.e., viewed from thesubstrate side). In these embodiments, the reflective layer 14 opticallyshields some portions of the interferometric modulator on the side ofthe reflective layer opposite the substrate 20, including the deformablelayer 34 and the bus structure 44. This allows the shielded areas to beconfigured and operated upon without negatively affecting the imagequality. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

With reference to FIG. 8, a cross section of the interferometricmodulator 12 is shown in an isolated view. In the exemplary illustratedembodiment, a conductor layer 100, having a fixed position, is depositedonto the glass substrate 20. As noted above, the fixed conductor layer100 is preferably partially reflective and transparent to desiredwavelengths of light, and can be made from, e.g., layers of ITO andchromium. A dielectric layer 102 is deposited onto conductor layer 100.The dielectric layer 102 can comprise silicon oxide, although otherdielectric materials, such as aluminum oxide, known in the art areequally applicable. For example, the layer 102 can comprise chargetrapping materials, and particularly materials that trap both positiveand negative charges, e.g., Al₂O₃, AlO_(x) (non-stoichiometric aluminumoxide), Si₃N₄, SiN_(x) (non-stoichiometric silicon nitride), Ta₂O₅ andTaO_(x) (non-stoichiometric tantalum oxide). The fixed conductor layer100 and the dielectric layer 102 form the optical stack 16.

Support posts 18 are provided to support the movable, reflective layer,or mechanical/mirror element, 14 a predetermined distance (in therelaxed mode) above the dielectric layer 102. The support posts 18 arepreferably formed of a stable material with sufficient structuralintegrity to support the movable reflective layer 14. For example, thesupport posts 18 can be fabricated from an organic material, such asphotoresist, or from spin-on glass. The movable layer 14 is preferablyformed of a flexible, conductive and highly reflective material, forexample, a metal such as aluminum, nickel, chromium or combinations oralloys thereof.

With continued reference to FIG. 8, a region 104 within the dielectriclayer 102 incorporates a charged species, preferably implanted ionsand/or dopants, to alter the optical response of the modulator 12. Ionsimplanted in the region 104 can be positively charged (p-type) ions ornegatively charged (n-type) ions according to the desired effect on thethreshold and/or hysteresis properties of the modulator 12. For example,potassium can be used as a positively charged ion and phosphorous can beused as a negatively charged ion. Examples of other positively chargedions, include without limitation, sodium ions and lithium ions.

It will be appreciated that the ions can be disposed in various otherpositions in the interferometric modulator 12. To prevent thedissipation of charge from the ions, the ions are preferably embeddedwithin a non-conducting material, such as a dielectric, or aresurrounded by or embedded within in a material surrounded by aninsulator. FIGS. 9-10 illustrate other non-limiting examples ofpositions for the ions. With reference to FIG. 9, a dielectric layer 106can be formed adjacent the movable layer 14 and the ion implanted region104 can be disposed within that layer 106. In such an arrangement, thedielectric layer 102, which typically serves to space and preventshorting between the movable layer 14 and the fixed conductive layer100, can optionally be omitted. Preferably, the dielectric layer 106 isformed of a flexible material that does not significantly impede themovement of the layer 14. As noted above, the dielectric layer 106 canbe formed of silicon oxide and aluminum oxide.

With reference to FIG. 10, where two dielectric layers, layers 102 and106 are provided, both layers can be implanted with ions to form ionimplanted regions 104 and 108. Depending on the desired effect, bothregions can be implanted with ions of the same polarity or with ions ofdifferent polarities. Implantation of both the layers 102 and 106 canincrease the effect of the implantation. For example, where regions 104and 108 are implanted with ions of different polarities, a constantlevel of attraction can be established between the layers 102 and 106,thereby reducing the actuation voltage in cases where the layers 100 and14 are wired to have the same polarity as the regions 104 and 108,respectively.

Along with changes in the actuation voltage, the introduction of chargedions between the movable layer 14 and the fixed layer 16 can shift theoptical response curves of the modulators 12. For example, the opticalresponse curves of FIGS. 11-13 plot the displacement of movable layer 14against the voltage applied to the movable layer 14 and the fixed layer16 of the interferometric modulator 12 of FIG. 8. In the positivevoltage region of the graph, the movable layer 14 is connected to avoltage source to generate a positive charge in that layer 14 and thefixed conductor layer 100 is connected to the voltage source to generatea negative charge in that layer 100. In the negative voltage region ofthe graph, the movable layer 14 is connected to a voltage source togenerate a negative charge in that layer 14 and the fixed conductorlayer 100 is connected to the voltage source to generate a positivecharge in that layer 100. At the bottom of the curves, the movable layer14 is in the relaxed position and, at the top of the curves, the movablelayer 14 is in the actuated position.

FIG. 11 illustrates the effect of implanting positively charged orp-type ions into the dielectric layer 102. The introduction of thepositively charged ions into the dielectric layer 102 causes the opticalresponse curves to shift to the right. In the positive voltage region,the p-type ions repel the positively charged movable layer 14, thusrequiring a larger voltage and greater electrostatic attraction to beapplied before the movable layer 14 can be made to collapse. In thenegative voltage region, the p-type ions attract the negatively chargedmovable layer 14, thus requiring a smaller voltage and lesserelectrostatic attraction to be applied before the movable layer 14 canbe made to collapse. Optical response curves 302 a and 302 b representthe optical response characteristics for an interferometric modulatorwithout ion implantation. The same interferometric modulator withpositively charged ions in the dielectric layer displays opticalresponse characteristics represented by optical response curves 304 aand 304 b. Because the level of charge introduced by the ions isconstant, the ions augment or reduce the net electrostatic attraction bythe same amount, so that both the positive and negative response curveswill shift to the right by the same amount. The amount of the shift inthe optical response characteristics is determined by the total chargeof the ions introduced into dielectric layer 102, which may beproportional to the amount of ions implanted.

FIG. 12 illustrates the effect of implanting negatively charged orn-type ions into the dielectric layer 102. The introduction ofnegatively charged ions into dielectric layer 102 causes the opticalresponse curves to shift to the left. In the positive voltage region,the n-type ions attract the movable layer 14 to the actuated position,while in the negative voltage region, the n-type ions repel the movablelayer 14 to maintain that layer in the relaxed position. Opticalresponse curves 312 a and 312 b represent the optical responsecharacteristics for an interferometric modulator 12 without ionimplantation. The same interferometric modulator 12 with negativelycharged ions in the dielectric layer displays optical responsecharacteristics represented by the optical response curves 314 a and 314b. As in FIG. 11, both the positive and negative response curves shiftto the left by the same amount and the amount of the shift in theoptical response characteristics is determined by the amount of ionsintroduced into dielectric layer 102.

It will be appreciated that interferometric modulators can be formedwith a hysteresis curve centered away from the zero voltage line. Forexample, the inteferometric modulators can be formed having a particularlevel of charge between the layers 14 and 16, even without ionimplantation. For example, structural defects or structuralmodifications in the dielectric layer 102 can result in such a charge.As a result of this charge, the hysteresis window for theseinterferometric modulators may not be centered relative to the zerovoltage line. Moreover, different interferometric modulators may exhibita different level of charge. The charges and the different levels ofcharges may adversely affect the behavior of the interferometricmodulators by reducing predictability and control over the actuation andrelease of the movable layers of the interferometric modulators.Advantageously, depending upon the charge already present, ionimplantation can allow the hysteresis behavior of the interferometricmodulators to be re-centered about the zero voltage line by, e.g.,neutralizing the already present charge. As a result, predictability andcontrol over the actuation and release of the movable layers of theinterferometric modulators can be increased.

While discussed above with reference to incorporated charged species inthe dielectric layer 102, it will be appreciated that similar affectscan be achieved by incorporation of charged species in the layer 106(when present, as illustrated in FIG. 9) or incorporation of chargedspecies in both the layers 102 and 106 (when present, as illustrated inFIG. 10). For example, charged species can be incorporated in the layer106 to achieve the effects illustrated in FIGS. 11 and 12. Where avoltage source is connected to the movable layer 14 and to the fixedlayer 16 to generate positive and negative charges in those layers,respectively, incorporation of a negative charged species in the layer106 will result in the rightward shift of the hysteresis curveillustrated in FIG. 11. Conversely, in a similar arrangement, but with apositive charged species in the layer 106, the hysteresis curve willshift to the left, as shown in FIG. 12.

Moreover, both the layers 102 and 106 can be provided in aninterferometric modulator and each can be incorporated with chargedspecies. For example, with the movable layer 14 and the fixed layer 16again configured to be positively and negatively charged, respectively,an effect similar to that illustrated in FIG. 11 can be achieved byincorporation of a positive charged species in the layer 102 and anegative charged species in the layer 106. In addition, the hysteresiscurves can be shifted to the left by incorporation of a negative chargedspecies in the layer 102 and a positive charged species in the layer106.

It will be appreciated that the effect of the incorporation of thecharged species can be reversed by reversing the polarities of thelayers 102 and 106. For example, if a certain arrangement of chargedspecies shifts the hysteresis curves to the left when the movable layer14 and the fixed layer 16 are connected to a voltage source to generatepositive and negative charges, respectively, in those layers, the samearrangement of charged species will shift the hysteresis curves to theright if the voltage source is connected to the layers 102 and 106 inreverse, i.e., so that the polarities in those layers is reversed.

With reference to FIG. 13, in some arrangements, interferometricmodulators can be formed to generate constructive interference centeredat a plurality of different frequencies to generate different perceivedcolors, e.g., two or more, or three or more different colors. Theseinterferometric modulators can be grouped to form, e.g., the individualred, green and blue picture elements of a display. It will beappreciated that the interference behavior of the interferometricmodulators is determined by the spacing between the movable layer 14 andthe fixed layer 16. Thus, interferometric modulators 100 a, 100 b, and100 c may be forming having a different spacing 110 a, 110 b, 110 cbetween the movable layers 14 a, 14 b, 14 c and the fixed layer 16,thereby allowing each of the different colors to be generated. Becauseof the different spacing, each interferometric modulator 100 a, 100 b,and 100 c can have a different actuation voltage and hysteresis curve.

Such a situation is illustrated in FIG. 14, which shows the hysteresiscurves, 322 a, 322 b and 322 c, for the three interferometric modulators100 a, 100 b, 100 c, respectively, each having a different spacingdesigned to give a different color. It will be appreciated that theinterferometric modulators in a display can be implanted with differentions and/or with different levels of ions. Advantageously, all or someof the interferometric 100 a, 100 b, 100 c modulators can be implantedwith ions to shift the curves 322 a-322 c so that they overlap. Byoverlapping the curves, the threshold and release voltages can be madesimilar, advantageously reducing the number of voltages needed foroperating the interferometric modulators and thus simplifying the driverand control systems associated with the interferometric modulators.

For example, in the arrangement shown in FIG. 14, the curve 322 b isused as a reference and the interferometric modulators associated withthe curves 322 a and 322 c are implanted with different ions, so thatthe curve 322 a (e.g., with a dielectric layer, between the layer 34 aand the conductor layer in the optical stack 16 of the interferometricmodulator 100 a, implanted with p-type ions) shifts to the right and thecurve 322 c (e.g., with a dielectric layer, between the layers 34 c andthe conductor layer in the optical stack 16 of the interferometricmodulator 100 c, implanted with n-type ions) shifts to the left, therebyallowing both to overlap the curve 322 b. As a result, all three curves322 a-322 c can advantageously be driven, in the positive voltageregion, using the same threshold and release voltages.

With reference to FIG. 15, flowchart 350 illustrates generally steps inthe formation of an interferometric modulator 12. A first conductivepart is formed 360, e.g., on a substrate, which can be, e.g., glass. Adielectric is formed 370 over the first conductive part and chargedspecies is added 380 into the dielectric layer.

While various process steps are illustrated as separate blocks in FIGS.15 and 16, it will be appreciated that the separate blocks do notindicate that the steps are necessarily temporally separated. Forexample, dielectric formation and charge addition can occursimultaneously, so that charged species, e.g., ions, are formed asas-deposited species in the dielectric. An example of a suitable processfor simultaneous dielectric formation and charge addition isco-sputtering, in which ionic species and dielectric precursors aresimultaneously sputtered on a substrate. In other embodiments, chargeaddition occurs after the formation of the dielectric. In this case,charge addition can be accomplished by various processes known in theart. In some embodiments, the ions can be implanted into the dielectricor can be diffused into the dielectric. For example, the dielectriclayer can be doped, e.g., diffusion doped.

With continued reference to FIG. 15, a second conductive part is formed390 over the ion implanted dielectric. It will be appreciated that thefirst conductive part can correspond to the one of the conductive layersof the interferometric modulator 12 (FIG. 8), e.g., the fixed conductivelayer 16. The second conductive part can correspond to the otherillustrated conductive layer of the interferometric modulator 12, e.g.,the movable layer 14.

With reference to FIGS. 8 and 16, flowchart 400 describes certain stepsof a fabrication sequence used to make the exemplary interferometricmodulator 12 illustrated in FIG. 8. The conductor layer 100, typicallycomprising ITO and chromium, is deposited 402 onto the substrate 20. Theconductor layer 100 is patterned and etched 404 to form rows of theinterferometric modulators 12. The dielectric layer 102 is deposited 406on conductor layer 100. This dielectric layer 102 can be formed of SiO₂,although other dielectrics compatible with the other materials andprocess steps for forming the interferometric modulator 12 can be used.A layer of photoresist is deposited and patterned 407 to provide a maskshielding some areas of the dielectric layer 102 and having openingsallowing for implantation in desired areas of the dielectric 102. Thecharged ions are implanted through the patterned photoresist and intothe dielectric layer 102, thereby forming the implanted regions 104 ofthe dielectric layers 102. The charge of the implanted ions and thedegree of the implantation is selected in accordance with the desiredeffect on optical response characteristics, as described above. Forexample, the polarity of the ions can be chosen based upon the directionin which a shift in a hysteresis curve (FIG. 11-13) is desired and thedegree of the implantation can be chosen based upon the magnitude of thedesired shift.

It will be appreciated that the ion implantation can disrupt thestructure of the dielectric. As a result, the ion implantation can befollowed by an anneal to reorient the dielectric structure, to improvethe optical characteristics of the implanted dielectric and to moreevenly distribute the ions within the dielectric.

It will be appreciated that, in some embodiments, no photoresist isneeded and the step 407 can be omitted if all interferometric modulators12 are to be uniformly implanted with the same ion or ions. In otherembodiments, the patterned photoresist allows interferometric modulators12 to be selectively implanted with ions, thereby allowing differentions to be implanted or different levels of implantation to be achieved.It will be appreciated that multiple photoresist depositions and/orpatterning steps can be used to selectively implant a plurality ofdifferent ions or to selectively implant different quantities of ions.For example, the photoresist can be deposited and patterned to implantions in some particular interferometric modulators 12, additionalphotoresist can be deposited and patterned to implant ions in otherinterferometric modulators 12, and so on. After the ion implantation,the photoresist is preferably removed.

Moreover, the charge incorporation can occur at a later step than thatillustrated. For example, the ion implantation can be performed afterdeposition 410 of a non-metal sacrificial layer (e.g., silicon),discussed below, and preferably before the formation of additional metallayers.

In step 410, a sacrificial layer (which will later be removed to formthe optical cavity of the interferometric modulator 12) is deposited.The sacrificial layer is formed of a solid material that can later beremoved, e.g., by etching, without disrupting the other materials of theinterferometric modulator 12. An example of a preferred material for thesacrificial layer is molybdenum. Other suitable sacrificial materialsinclude silicon and tungsten, which advantageously can also beselectively or preferentially removed using XeF₂ without etchingaluminum or silicon oxide. The sacrificial layer is patterned and etched412 to provide voids into which materials to form the support posts 18will be deposited. In step 414, the post material is deposited, therebyforming the support posts 18. The post material can be, e.g.,photoresist or some other organic compound or spin on glass. In step416, a mechanical/mirror film is deposited. As noted above, the film canbe made of, e.g., aluminum or other flexible metals. In step 418, themechanical film is patterned and etched to form the mechanical/mirrorlayer 14. The sacrificial layer is then removed 420.

Interferometric modulators according to the preferred embodiments offernumerous advantages. For example, charge incorporation allows theactuation voltage and/or hysteresis curve for a particularinterferometric modulator to be shifted as desired. As a result, it ispossible to reduce the voltages required to drive the interferometricmodulators, thereby lowering the power requirements and powerconsumption of displayers utilizing the interferometric modulators. Inaddition, hysteresis curves can be shifted by charge incorporation tocenter the curves about the zero voltage line. This can be achieved, forexample, by neutralizing charges that may form in the dielectric layerof between the conductive layers of the interferometric modulators.Centering the hysteresis curves can make control over the states of theinterferometric modulators more predictable, e.g., by setting theactuation voltages at expected values. Moreover, in cases in whichmultiple interferometric modulators, each naturally having a differentactuation voltage and shifted hysteresis curves, are present, some orall of the interferometric modulators can be doped so that thehysteresis curves substantially overlap. As a result, the actuationvoltages and the release voltages of each of the interferometricmodulators overlap, thereby reducing the number of different voltagesthat is generated to control the interferometric modulators. Thus, thedriver and control systems can be simplified.

While the charged species are discussed above as “implanted ions,” itwill be appreciated that the charged species can be any charged speciesincorporated in a material disposed between the movable conductive layerand the fixed conductive layer. In other embodiments, the chargedspecies can simply be deposited on a dielectric substrate and preferablyhas a charge as deposited. The dielectric layer, while preferablydisposed on a conductive layer for simplicity of fabrication andstructure, can be spaced from the conductive layers. Moreover, whilediscussed as having one movable and one fixed conductive layer for easeof description and illustration, in some embodiments, the positions ofthe movable and fixed layers can be reversed from that illustrated, orboth layers can be made to move.

Accordingly, it will be appreciated by those skilled in the art thatvarious other omissions, additions and modifications may be made to themethods and structures described above without departing from the scopeof the invention. All such modifications and changes are intended tofall within the scope of the invention, as defined by the appendedclaims.

1. A micro electromechanical system, comprising: a conductor layer; amechanical layer separated by a cavity from the conductor layer andconfigured to move relative to the conductor layer; and a charged layercomprising an incorporated charged species, the charged layer disposedbetween the conductor layer and the mechanical layer, further comprisinga plurality of interferometric light modulating devices, wherein some ofthe interferometric light modulating devices comprise the charged layerand other of the interferometric light modulating devices lack thecharged layer.
 2. The microelectromechanical system of claim 1, whereinthe charged layer is connected in fixed relation to one of the conductorlayer and the mechanical layer.
 3. The microelectromechanical system ofclaim 2, wherein the charged layer is immovable and directly overliesthe conductor layer.
 4. The microelectromechanical system of claim 1,wherein the charged layer is an ion implanted dielectric layer.
 5. Themicroelectromechanical system of claim 1, wherein the incorporatedcharged species are as-deposited ions in the dielectric layer.
 6. Themicroelectromechanical system of claim 1, wherein the incorporatedcharged species is potassium or phosphorous.
 7. Themicroelectromechanical system of claim 1, wherein the incorporatedcharged species is a positively-charged ion chosen from the groupconsisting of sodium ions an.
 8. The microelectromechanical system ofclaim 1, wherein the mechanical layer is attached to and disposed infixed relation to a mirror spaced from the mechanical layer, wherein themechanical layer is configured to move the mirror relative to thecharged layer.
 9. The microelectromechanical system of claim 1, whereinthe plurality of interferometric light modulating devices form adisplay. further comprising: a processor that is in electricalcommunication with the display, the processor being configured toprocess image data; and a memory device in electrical communication withthe processor.
 10. The microelectromechanical system of claim 9, furthercomprising: a driver circuit configured to send at least one signal tothe display.
 11. The microelectromechanical system of claim 10, furthercomprising: a controller configured to send at least a portion of theimage data to the driver circuit.
 12. The microelectromechanical systemof claim 9, further comprising: an image source module configured tosend the image data to the processor.
 13. The microelectromechanicalsystem of claim 12, wherein the image source module comprises at leastone of a receiver, transceiver, and transmitter.
 14. Themicroelectromechanical system of claim 9, further comprising: an inputdevice configured to receive input data and to communicate the inputdata to the processor.
 15. The microelectromechanical system of claim 1,wherein the some of the interferometric light modulating devices haveconductor layers and mechanical layers configured for producingconstructive interference predominately at one frequency and wherein theother of the interferometric light modulating devices have conductorlayers and mechanical layers configured for producing constructiveinterference predominately at one or more other frequencies, wherein thesome of the interferometric light modulating devices and the other ofthe interferometric light modulating devices are grouped to form pixelsin the display.
 16. A microelectromechanical system, comprising: aplurality of interferometric modulators, each interferometric modulatorcomprising: a conductor layer; a mechanical layer separated by a cavityfrom the conductor layer and configured to move relative to theconductor layer; and a charged layer comprising an incorporated chargedspecies, the charged layer disposed between the conductor layer and themechanical layer, wherein at least some of the interferometric lightmodulating devices comprise different incorporated charged species thanother of the interferometric light modulating devices.
 17. Themicroelectromechanical system of claim 16, wherein the at least some ofthe interferometric light modulating devices comprise positively chargedincorporated charged species and the other of the interferometric lightmodulating devices comprise negatively charged incorporated chargedspecies.
 18. A microelectromechanical system, comprising: a conductorlayer; a mechanical layer separated by a cavity from the conductor layerand configured to move relative to the conductor; a charged layercomprising an incorporated charged species, the charged layer disposedbetween the conductor layer and the mechanical layer; and an othercharged layer spaced from the charged layer by the cavity, the othercharged layer comprising additional incorporated charged species. 19.The microelectromechanical system of claim 18, wherein the additionalincorporated charged species of the other charged layer has an oppositepolarity to the incorporated charged species of the charged layer. 20.The microelectromechanical system of claim 18, wherein the additionalincorporated charged species of the other charged layer has a samepolarity as the incorporated charged species of the charged layer.